1. Field of the Invention
This invention relates generally to boiling water nuclear reactors and more particularly, to repairing or sealing leaking elongate hollow members such as control rod drive housings and stub tubes in a nuclear reactor pressure vessel of such reactors.
2. Related Art
Boiling water nuclear reactors typically include a reactor core located within a reactor pressure vessel (RPV). A known RPV includes a substantially cylindrical shell. The shell, for example, can be about twenty feet in diameter and about seven inches thick.
The cylindrical shell is closed at its top end by a removable top head. The top head is removable so that components, such a fuel bundles, located in the RPV can be accessed. The RPV cylindrical shell is closed at its bottom end by a dome shaped bottom head assembly welded to the shell.
A plurality of openings are formed in the bottom head dome so that components, such as control rod drive assemblies, can extend within the RPV. Typically, a substantially cylindrical stub tube having a bore extending therethrough is welded to the bottom head dome and the tube bore aligns with an opening in the bottom head dome. The cylindrical stub tube typically is fabricated from a corrosion resistant material such as stainless steel or Nixe2x80x94Crxe2x80x94Fe.
As an example, for a control rod drive assembly, the control rod drive housing, e.g., a tube, is inserted through the bottom head dome opening and stub tube bore, and the housing extends into the RPV. The control rod drive (CRD) housing is welded to the stub tube to maintain the housing in the desired position. The stub tube thus serves as a transition piece between the bottom head dome, which typically is fabricated from low alloy steel, and the CRD housing, which typically is fabricated from stainless steel such as 304 stainless steel with a high carbon content.
Inter-granular stress corrosion cracking (IGSCC) is a known phenomenon occurring adjacent to stub tube welds connecting the bottom head dome to the stub tube and connecting the stub tube to the CRD housing. Particularly, the stub tube welds are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. Such stresses may, at times, cause cracks adjacent the stub tube welds.
If stress corrosion cracks adjacent stub tube welds are not sealed, such cracks cause potential leakage paths between the stub tube and the bottom head dome, and the stub tube and the CRD housing, respectively, which is undesirable. Accordingly, upon detection of any such cracks, it is desirable to re-seal the control rod drive housing, for example, to the bottom head dome.
Type 304 stainless steel stub tubes in some plants have become furnace sensitized as a result of vessel post weld heat treatment. This has left the stub tube in an inter-granular stress corrosion cracking (IGSCC) susceptible state, and has led to leaking cracks. Cracking has been observed in the heat affected zone of the stub tube at the CRD housing attachment weld of this susceptible material. This results in a reactor coolant leakage path to the undervessel area. Restoration of the defective area is virtually impossible due to the location of the stub tubes and the existing material condition.
One known method of repairing or re-sealing CRD housings within the bottom head dome includes completely replacing the stub tube and CRD housing. This method, however, is time consuming, tedious, and expensive. Particularly, the housing and associated stub tube are partially cut-off and the material remaining in the bottom head assembly is inspected to ensure that such material can be welded without damaging the bottom head. A weld build-up is then formed over the remaining material and machined so that a new stub tube can be welded to the weld build-up. Several weeks can be required to perform the replacement process of just one penetration tube. Moreover, much of the replacement work must be performed within the RPV, which requires completely unloading the RPV and is undesirable.
Another known method of repairing or re-sealing a CRD housing within the bottom head dome includes welding a sleeve to the CRD housing and the stub. This method, however, only addresses stress corrosion cracks adjacent the interface between the stub tube and the CRD housing. Moreover, installing the sleeve must be performed entirely within the RPV which, as explained above, is undesirable.
Yet another known method of repairing or re-sealing a CRD housing within the bottom head dome includes rolling the CRD housing into the bottom head dome. While this method is quicker than replacing the stub tube and CRD housing, rolling the CRD housing into the bottom head dome does not create as tight a seal as a weld between the CRD housing and the dome. Moreover, the rolled CRD housing may become separated from the bottom head dome after continued RPV operation, and must then be re-rolled. Re-rolling a CRD housing, however, often is neither desirable nor practical.
Still another known method of repairing or re-sealing a CRD housing within the bottom head dome includes removing and replacing a lower portion of the CRD housing within the bottom head dome. Particularly, a lower portion of the CRD housing is cut-off so that an upper portion of the CRD housing remains inserted in an opening in the bottom head dome and welded to the stub tube. The bottom head dome is then cleaned, and the lower end of the remaining CRD housing upper portion is machined so that a replacement bottom portion of CRD housing can be welded to the remaining upper portion. The replacement bottom portion of CRD housing similarly is machined so that it can be welded to the remaining upper portion. The replacement bottom portion of CRD housing is then inserted into the bottom head dome opening and positioned adjacent the remaining upper portion. The replacement bottom portion and the remaining upper portion of CRD housing are then temper bead welded to each other and to the bottom head dome. Temper bead welding the remaining upper portion to the replacement lower portion and the bottom head dome has the undesirable effect of causing high stresses because of thermal growth mismatch between the CRD housing and the bottom head dome, which are fabricated from different materials. Such temper bead welding also has the undesirable potential effect of trapping water within a leakage path and in contact with the weld between the remaining upper portion and the bottom head dome.
It would be desirable, therefore, to provide a method for repairing and/or sealing a CRD housing within the bottom head dome which can be more easily and quickly performed than known sealing methods. It further would be desirable to provide such a method which reduces stresses caused by thermal growth mismatches between the CRD housing and the bottom head dome.
The method in accordance with the invention, in one embodiment, facilitates repairing and/or re-sealing a substantially elongate hollow member such as a control rod drive (CRD) housing within a reactor pressure vessel of a nuclear reactor more quickly and easily than known methods. The method eliminates leaks due to cracking in a heat affected zone of a stub tube at a CRD housing re-attachment weld and restores the pressure boundary. The reactor pressure vessel includes a bottom head dome having at least one opening extending therethrough, a substantially hollow stub tube, and a control rod drive housing. A lower portion of the stub tube is welded to the bottom head dome with a lower stub tube weld so that a bore extending through the stub tube is substantially aligned with the bottom head dome opening. The upper portion of the stub tube is welded to the CRD housing with an upper stub tube weld so that the CRD housing extends through and is secured within the bottom head dome opening and the stub tube bore.
The method includes cutting the CRD housing at a location below the upper stub tube weld to separate an upper portion of the CRD housing from a lower portion of the CRD housing. For example, the existing lower portion of the CRD housing is severed below a stub tube attachment weld. The existing lower portion is not removed; however, but remains in place and in substantial alignment with the upper portion of the CRD housing. After machining, cleaning and buffering of the cut lower portion, a lower portion member, which may be the lower portion that was cut, is re-attached to the reactor vessel at a point below potential differential leakage paths along the CRD housing. The existing lower portion is then secured to the bottom head dome without also being secured to the CRD upper portion. Specifically, the existing lower portion is temper bead welded to a sidewall of the bottom head dome opening. The heat affected zone (e.g., at the temper beaded re-attachment weld) is covered with a corrosion resistant material to prevent future leaking, and to protect the weld heat-affected zone from IGSCC.